The present invention relates generally to laser radar system, and more particularly to continuous wave (CW) laser radar systems.
As is known, CW laser radar systems have a wide variety of applications, one of such applications being to provide range tracking of a target as well as imaging and classification of such target, that is, providing details of the shape of the target to permit such target to be identified. Such application requires the CW laser radar to provide precise target range information. A conventional CW laser radar utilized for such application provides amplitude modulation (AM) of the transmitted CW laser beam by passing the CW beam through a modulator, such as an electro-optic crystal, biased with a time-varying signal at the AM frequency. Target range is measured by determining the phase shift of the AM envelope of the target reflected return signal relative to a reference AM envelope signal. Since phase shift measurement accuracy of a few degrees may be conventionally obtained, and since very high modulation frequencies (i.e., on the order of 1 MHz) may be achieved using conventional AM electro-optic crystals, amplitude modulated CW laser radar systems are capable of measuring target range quite accurately.
However, amplitude modulated CW laser radar systems with high AM frequencies have poor target resolution. That is, it is difficult to resolve a selected target from other, unselected targets or from atmospheric clutter such as near-field clutter through the use of amplitude modulation. Such poor resolution is due to the fact that scatterers of energy at different ranges (for example, selected and unselected targets, atmospheric clutter, etc.) will return signals to the radar system which have AM envelopes at different relative phases with respect to the reference AM envelope. Thus, the measured phase shift between the return signals and the reference signal is an average phase shift contributed to by several sources other than the selected target, thereby substantially preventing the selected target from being distinguished from other targets or from atmospheric clutter. Additionally, since the AM frequency is typically selected to be relatively high in order to obtain accurate target range measurements, the AM waveform has a relatively short ambiguous range interval. The AM frequency could be lowered to increase the ambiguous range interval, but the AM waveform would have poor accuracy and would still have poor target resolution.
Another CW laser radar system utilizes frequency modulation (FM) rather than AM to provide target range information. The CW signal is periodically "chirped" in frequency at a predetermined repetition rate and by a predetermined amount to thereby impose FM on the CW beam. The repetition frequency of the FM modulation (i.e., the FM modulation frequency) is typically much lower than the modulation frequency in conventional AM systems, leading to long ambiguous range intervals. Further, the FM waveform provides relatively high target resolution according to the FM modulation frequency. Thus, the FM radar system is capable of distinguishing a selected target from other targets and from near-field atmospheric clutter. However, due to inherent FM bandwidth and chirp limitations, inaccuracies are experienced in imposing FM on a CW laser beam. Thus, the accuracy to which target range may be measured with an FM-CW laser beam may not be sufficient to provide accurate information on the details of the target, thereby decreasing the probability of measuring target characteristics and/or degrading the capability of the laser radar system to identify and classify the target.